Compared with other adhesives, hot-melt adhesives are advantageous in that they can be applied to a broad range of adherends, feature a very high rate of adhesion, and are of low intoxication and hazard potential and more economic and, therefore, have been used broadly in such applications as bookbinding, packaging, carpentry, shoemaking, fabric bonding and so forth.
Hot-melt adhesives have to satisfy various performance requirements such as bonding strength, meltability, heat resistance, cold resistance and flexibility. The base polymers used for such hot-melt adhesives are generally ethylene-vinyl acetate copolymer, polyethylene, polypropylene, polyamide and polyester resins.
As the hot-melt adhesives for use in fiber oriented fields such as bookbinding, bag making, carton making, woodworking, etc., ethylene-vinyl acetate copolymer having satisfactory adhesiveness, workability and flexibility characteristics is in widespread use.
However, since ethylene-vinyl acetate copolymer is hydrophobic, hot-melt adhesives containing this copolymer as the base polymer have the drawback that when, for instance, stock paper carrying such an adhesive is treated with water for regeneration, the paper is not uniformly dissolved or dispersed to cause troubles.
Recently, interest is mounting in hot-melt adhesives using water-soluble polyvinyl alcohol resin as the polymer base.
However, since vinyl alcohol resin is high-melting and shows a high melt viscosity at low temperature, this resin alone is not suitable for hot-melt adhesive use. Therefore, some plasticer such as glycerin, ethylene glycol or the like is added to the resin but the mere addition of a plasticizer is not successful for reducing the melting point and melt viscosity to the necessary levels. For this reason, hot-melt adhesives based on polyvinyl alcohol are of necessity used at high temperature (not less than 180.degree. C) but the use of such a high temperature for melting tends to cause a thermal degradation of the polyvinyl alcohol resin and lower the compatibility of the resin with other ingredients.
In addition, since polyvinyl alcohol resin has the property to become brittle at low temperature, products assembled with a PVA-based hot-melt adhesive are liable to fail at the joints especially in cold districts and are, therefore, of low market value.
Moreover, compared with the more conventional hot-melt adhesives, PVA-based adhesives absorb moisture in the highly humid environment to deteriorate in bond strength and blocking resistance.
Japanese Patent Publication No. 56196/1985 describes a water-soluble/-dispersible hot-melt adhesive composition comprising a polyvinyl alcohol modified with an olefinsulfonic acid or an alkali metal salt thereof and a plasticizer.
However, this water-soluble/-dispersible hot-melt adhesive is also deficient in bond strength and blocking resistance under high-humidity conditions.
It is an object of this invention to provide a water-soluble/-dispersible polyvinyl acetate resin composition which is low-melting and shows a low melt viscosity in a low-temperature region, with reduced brittleness at low temperature and insuring satisfactory bond strength and blocking resistance under high-humidity conditions.
It is a further object of the invention to provide a hot-melt adhesive comprising said resin composition and a method for its manufacture.